1. Field of the Invention
The invention relates to an optical disk apparatus, and more particularly, to an optical disk apparatus of phase change type utilizing transition from an amorphous state to a crystalline state.
2. Related Art
There has hitherto been known an optical disk apparatus of phase change type which effects transition of a recording film from an amorphous state to a crystalline state and records data by utilization of a change arising between the reflectivity of the film in the amorphous state and that of the film in the crystalline state. More specifically, the entire surface of the recording film is crystallized as an initial state through heating. A laser beam of recording power is radiated onto the recording film, thereby heating the film to a melting point or more. The film is then rapidly cooled to bring the crystalline state to the amorphous state, thereby recording data. A laser beam of erasing power is radiated onto the recording film to a crystallization temperature or more. The film is then gradually cooled so as to return the amorphous state to the crystalline state, thereby erasing the data.
In such an optical disk apparatus of phase change type, overwriting of data is also performed by modulation of the power of the laser beam. Specifically, overwriting operation is realized by superimposing a recording power pulse on an erasing power pulse at the time of recording of data, by erasing existing recorded data through use of the preceding erasing power pulse, and by recording data through use of the subsequent recording power pulse. Normally, the recording power is optimized through OPC (Optimum Power Control) which uses a predetermined area of an optical disk; that is, a PCA (Power Calibration Area), in consideration of variations in the sensitivity of the recording film of the optical disk or other factors. The erasing power is set so as to assume a constant proportion to the optimized recording power.
With a view toward improvement in a signal-to-noise ratio, overwriting requires complete erasure of existing recorded data through use of the laser beam of erasing power. Consequently, in addition to the method for setting the erasing power so as to assume a constant proportion to the recording power, a method for optimizing the erasing power itself in accordance with an optical disk has also been proposed. In short, there has been proposed a method for setting the erasing power independent of the recording power.
For example, Japanese Patent Laid-Open Publication No. Hei 10-188286 describes a method for setting optimal erasing power by recording a test signal on a certain track of an optical disk, tracing the track with a DC laser beam of erasing power (Pe), and examining the level of erasure of a marked portion (corresponding to an amorphous area) of the reproduced signal. If the erasing power is too low, unerased data still remain on the track. In contrast, if the erasing power is too high, a transition to the amorphous state arises again. For these reasons, optimal erasing power is selected from the range of intermediate power between the too-low erasing power and the too-high erasing power.
However, demand for high-speed recording has recently been growing, and there arises a problem of optimization of the erasing power alone sometimes failing to meet such a demand for high-speed recording. For example, in relation to an optical disk apparatus which is of quadruple-speed as defined by the specifications, many users desire to perform recording operation at a speed faster than quadruple-speed (4×); for example, octuple-speed (8×) or decuple-speed (10×). Recording of data at a speed faster than quadruple-speed is favorable for the optical disk apparatus. When overwriting is performed through use of optimized erasing power, erasure of existing recorded data at quadruple-speed may be ensured, but thorough erasure of data at a speed higher than quadruple-speed is not ensured. Unerased data may arise, which in turn degrades recording quality. The reason for this is that the higher a recording speed (i.e., the higher the rotational speed of a disk), the shorter the time during which a recording film is to be heated, thereby posing difficulty in crystallization of the film, which would otherwise be caused when the film is held at a temperature higher than or equal to the crystallization temperature for a given period of time.